Laser-based tools have become the standard for treating and diagnosing quite a number of ophthalmic conditions, such as the complications associated with diabetes, glaucoma, and aging. Lasers are also used, amongst other things, to seal retinal holes and detachments by welding them back together. One element of these types of laser surgery is the use of the Retinal Pigmented Epithelium (RPE) as a target for absorbing the laser energy. The RPE is a single cellular layer that separates the photoreceptors from their blood supply in the choroid. Retinal laser surgery can be classified into two distinct types of treatment. Some treatments rely on thermal damage to the neuroretinal layer (such as retinal welding), and others seek to minimize damage to the neuroretina (such as microphotocoagulation, or selective RPE therapy (SRT)). However, both these treatment types are often typically referred to simply as “photocoagulation” (PC).
PhotoCoagulation (PC) has been established as a standard for treating a wide variety of retinal disorders, for example, in the condition known as proliferative diabetic retinopathy. In this condition, abnormal new blood vessels develop from the optic nerve area or elsewhere in the retina and frequently lead to severe visual loss from hemorrhage and other complications. PC is typically applied extensively from near the optic nerve margin to the periphery of the retina, while sparing the central macular area, which has the highest density of photoreceptors, and is functionally important for seeing detail.
In conventional PC, the lightpulses used to implement the PhotoCoagulation are usually of the order of 50-100 ms with fluences in the order of ˜4 J/mm2. These parameters allow for considerable heat flow from the melanosomes (which are the pigmented organelles that contain the RPE's melanin) during irradiation. This heat flow can often damage the surrounding structures, such as the photoreceptors. This is undesirable, as amongst other things, it may lower one's visual acuity by creating blindspots known as “scotomas”.
More recently a new approach to PC therapy has emerged, which is often referred to as “MicroPhotoCoagulatian,” (MPC) or “Selective RPE Treatment” (SRT). The main difference between early SRT techniques and PC is that SRT typically uses laser pulses of short duration (for example, 10−6s) so that the heat generated by their absorption in the RPE generates high temperatures only in the melanosomes, while still instigating a generalized retinal wound healing response.
Original MicroPhotoCoagulation research includes work by Birgruber, Roider et al. in Lubeck, Germany, using pulsed lasers delivering relatively large spots. These pulses, however, are typically expensive to manufacture.
Further work on MicroPhotoCoagulation has been done by Lin et al. at the Wellman Laboratories of Photomedicine in Boston, using a fiber-coupled, scanned continuous wave (CW) 532 nm laser with fluences of ˜0.3 J/cm2 that is delivered with microsecond dwell times. This method, however, delivers round spots that may result in uneven scanning of the target tissue due to the difference in dwell times across the beam.